1. Field of the Invention
The present invention relates to an intrusion detector system, and, more particularly, to an apparatus for enabling and disabling a lookdown zone mask in an intrusion detector unit.
2. Description of the Related Art
Passive infrared (PR) motion sensors are well-known. PIR motion sensors are useful due to less susceptibility to false alarms as compared to other technologies, for example, ultrasonic and microwave motion sensors. PIR motion sensors have no active component which is radiated from the unit, such as sound or radio waves, for their operation. In general, a PIR motion sensor is typically mounted on a wall or ceiling and protects a room or other area by imaging multiple areas of the room onto an infrared detector. The output of the detector is amplified and processed for alarm output from the motion sensor.
Prior PR motion sensors use a single Fresnel lens array or minor array to focus multiple areas of the room onto the detector. Although most of the areas of the room can be protected with this arrangement, the area directly below the sensor is typically out of range of the detector. To correct this situation, a “lookdown zone” capability was included in the motion sensor to add protection to the area directly below the sensor. The lookdown zone capability may be accomplished by adding a Fresnel lens near the detector which allows imaging of a small area, located almost directly below the sensor, onto the detector.
Referring to FIGS. 1 and 2, for example, intrusion detector unit 20 is shown and may include printed circuit board (PCB) or substrate 22, detector 24, lens assembly 26, and lens 28. PCB 22 includes circuitry (not shown) for processing of an alarm output from detector 24. Lens assembly 26 may include mounting structure 30 having two side supports 31. Mounting structure 30 is connected to PCB 22 and is operable to mount detector 24 and lens 28 in a given relationship relative to each other. Lens 28 provides the lookdown zone for unit 20, and, in an exemplary embodiment, lens 28 is a Fresnel lens, i.e., lens 28 is flat on one side facing detector 24 and ridged on the other side facing away from detector 24, as is commonly known.
While the arrangement of FIGS. 1 and 2 offers better protection than an intrusion detector system without a lookdown zone capability, certain environments are more susceptible to false alarms when the lookdown zone is enabled in the motion sensor. For example, pets can trigger the alarm in a residential setting and rodents can trigger the alarm in a commercial setting. To prevent such unwanted false alarms, the lookdown zone may incorporate a mask to disable the lookdown zone. The mask may be a colored masking tape which is die cut to fit over the lens used for the lookdown zone. Alternatively, the mask may be a removable, adhesive paper that covers the lens used for the lookdown zone. The mask may be installed during manufacture of the intrusion detector unit and remain until the user desires removal thereof if there is only a small chance of false alarms occurring and the end user wants the additional coverage provided by the lookdown zone.
Referring to FIG. 3, for example, unit 20 may include lens mask or cover 32 which may be attached to lens 28 prior to installation of unit 20 in a desired location. Mask 32 may be attached to lens 28 by an adhesive to facilitate later removal of mask 32 from lens 28 if desired by an installer or end user. If mask 32 is removed from lens 28, the lookdown zone is enabled because lens 28 permits radiant energy to pass through to detector 24, and if mask 32 remains attached to lens 28, the lookdown zone remains disabled because mask 32 covers lens 28 and prevents radiant energy to pass through mask 32 to detector 24. One example of an adhesive mask is shown in U.S. Pat. No. 5,026,990.
Once the installer or end user has removed and discarded the tape or paper used to mask the lookdown zone, however, the lookdown zone cannot be remasked should the environment change and/or false alarms become a problem without risking damage to other components of the intrusion detector unit, for example, the printed circuit board. Furthermore, if the installer does not remove the mask during installation, an end user must later disassemble the intrusion detector unit and remove the mask to enable the lookdown zone at a later time. Such an operation similarly risks damage to the printed circuit board and other internal components of the intrusion detector unit. Damage to any internal components of the unit and/or disassembly and reassembly of the unit may undesirably make the intrusion detector unit less effective, completely inoperable, or more susceptible to tampering by an intruder.
U.S. Pat. No. 6,987,267 discloses a lens blind or door for a motion detector. The blinds or doors are rotatably mounted within a housing and serve to limit, enlarge, or otherwise control the detection angle of the PIR sensors. The blinds include control knobs by means of which the blinds can be rotated to define the desired detection angle. Opaque doors are used to block or limit the detection angle of the sensor. The doors are not used to either enable or disable a lookdown zone, but are rather to customize the detection angle of the motion detector.
U.S. Pat. No. 5,015,994 discloses a security light controlled by a motion detector which includes a vision extender. The vision extender permits the user to select the field of view of the motion detector. By locating the vision extender at different points along the arc of a lens, optimal viewing can be achieved for a particular location of the motion detector.
U.S. Pat. No. 5,818,337 discloses a masked passive infrared intrusion detection device wherein a masking element is used to block or mask a part of the infrared radiation from a lookdown space. By means of this arrangement, pyroelectric detector elements are shaded from viewing the lens element on the opposite side by a different amount. When an intruder enters into the detection pattern, the intruder will be visible to a larger extent by one of the single detector elements, thereby improving the effectiveness of the detector.
What is needed in the art is a lookdown zone mask for an intrusion detector that permits repeated switching from an enabled to a disabled lookdown zone.